This invention relates to micro- and nano-switching actuators employing magnetostrictive films.
Switches, adaptive optics, and MEMS devices require actuators for their operation. There are several known actuator technologies that can be used for these applications. Thermal actuation utilizes bimorphs, laminate structures made up of two materials having dissimilar coefficients of thermal expansion. As bimorphs are heated, the differential thermal expansion causes a bending motion. Thermal actuation, however, is plagued by slow response and control difficulties when operating conditions of the system are not isothermal. Further, there is no opportunity for a long-term shape memory effect.
Electrostatic actuation is another modality. Voltages are applied to structures to cause them to move. Electrostatic actuation is the most common actuation method for MEMS devices. While electrostatic actuation is very well understood, there are several disadvantages. First of all, electrostatic actuators require significant applied voltage requiring careful design to avoid an instability that occurs when the applied voltage is increased beyond a critical value. This instability is known as “snap-down”. Once the critical value is exceeded, there is no longer a steady-state configuration of the device such that an actuated portion and a substrate remain separated. Further, the control of electrostatic actuators requires wires that impose difficult space constraints as devices shrink in size and there is no opportunity for remote control or for long-term shape memory effect.
Piezoelectric actuation is another very common actuation modality. As is well known, piezoelectric materials deform under an applied voltage to provide actuation. These actuators require a significant applied voltage and cannot be actuated remotely. There are also space constraints because of the need for wires and/or electrical traces. As with electrostatic actuation, piezoelectric methods cannot provide for long-term shape memory effect. Some piezoelectric materials are polymers that require moisture to operate. Their applicability is therefore limited and varies with environmental conditions.
Shape memory alloys have also been used for actuators. Shape memory materials undergo a martensitic transformation when heated, cooled or under an applied magnetic field. Such actuators offer slow response, can be difficult to control when operating conditions are not isothermal and impose constraints on operating temperatures.
Microfluidics flow control is another area requiring micro and/or nano-control. The most common forms of flow control in microfluidic devices are either passive, allowing droplets of fluid to flow through channels and mix diffusively, or based on pneumatic valves that flex under pressure. The advantage of passive flow control is that it is very robust because there are no moving parts and no active intervention. A main disadvantage, however, is that diffusive mixing is slow. Mixing of fluid streams in such a device can be on the order of minutes which severely limits through-put of material.
A pneumatic valve system, on the other hand, enables channel designs with rapid mixing. Such pneumatic valves are often made of an elastomer such as poly(dimethylsiloxane) (PDMS). Bladder-like areas of PDMS are inflated and deflated using applied pressure. There are, however, severe design restrictions. For example, each valve or array of valves connected to a single pressure manifold is constrained to open and close as a function of pressure drop. This means that if the channels to be controlled are the same length, all valves will actuate at the same time. To circumvent this limitation, channel lengths must be either designed as a function of pressure drop rather than optimized for the application, or separate pressure manifolds must be used for different channels requiring additional connections to the outside world and constraining the size of the device. See, for example, “Protein Crystalization Enabled by the NanoFlex™ Valve,” Fluidigm Corporation, 2004, http://www.fluidigm.com.
Parallel-plate electrostatic valves have also been used for microfluidics flow control. These are typical MEMS actuators that bend under an applied voltage. See, for example, “Microvalves,” Boston Micromachines Corporation, 2004, http://www.bostonmachines.com/products/micro valves.htm. Parallel-plate electrostatic valves are actuated electrostatically and therefore require on-chip power and there is no long-term memory so that power must be applied continuously to keep a valve actuated. Further, a parallel-plate electrostatic actuator is susceptible to “snap-down” as discussed earlier. This phenomenon severely restricts the range of stable operation of such valves and places size restrictions on the device.
Space inflatable structures such as solar sails, radar-based applications and optical mirrors require actuators for shape control. For externally reacted actuators, shape change is effected by positioning macroscopic actuators of any type behind a membrane of the structure. Precise control is effected by pushing on the membrane with the appropriate actuator in the appropriate location in response to a control signal. Alternatively, internally-reacted actuation is achieved by integrating into or onto the membrane a material that changes shape in response to a stimulus. See, E. M. Flint and K. K. Denoyer, “Approach for efficiently evaluating internally reacted global shape control actuation strategies for apertures,” 44th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, and Materials Conf, Norfolk, Va. April 2003.
Space inflatable structures have also been controlled with piezoelectric patches, small localized patches of material such as lead zirconate titanate (PZT) attached to polymer membranes in a selected control pattern. The PZT is then actuated by an applied voltage.
Shape control for space inflatable structures is also known using photostrictive materials actuated by shining light onto them such as from a laser. Photostrictive materials have disadvantages because relaxation response is very slow, and for materials of faster relaxation response, the achieved strain is very small. Further, localized heating resulting from the applied light may overwhelm the photostrictive response. Finally, photostrictive actuators need to be within line of sight of the controlled light source.
Shape memory alloys have also been used for space inflatable structure applications and are usually actuated by temperature change.
Electroactive polymer (EAP) bimorphs are also known for this application. These actuators are polymeric piezoelectric materials that can be poured onto a membrane and cured. An example of such a material is polyvinylidene fluoride (PVDF). It is often patterned to make a control pattern similar to other methods. See, D. M. Sobers, G. S. Agnes, D. Mollenhauer, “Smart Structures for Control of Optical Surfaces,” 44th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, and Materials Conf., Norfolk, Va. April 2003.
There is therefore a need for actuators for use in the above-mentioned applications that have fewer disadvantages than the actuators known in the prior art.